Aliasing happens when an analog-to-digital converter samples a signal containing frequencies above half the sample rate (Nyquist frequency). Those high frequencies 'fold back' into the audible range as aliases — inharmonic, metallic-sounding artifacts.
Anti-aliasing filters (low-pass filters before the ADC) prevent aliasing by removing frequencies above the Nyquist limit. Modern audio interfaces include oversampling to push aliasing artifacts far above the audible range.